The present invention relates to a process for charging a structure comprising an insulating body and a charging device for such a structure. Control of conditions for charging an insulating body, in other words knowledge of the quantity of the charge and the distribution of the charges then makes it possible to study the potential decay phenomenon from when it starts, and the potential return with time after the structure has discharged. These studies then determine the electrical properties of the body such as the electronic mobility of the insulating material, its conductivity and its dielectric constant. Knowledge of these properties is essential to determine the aptitude of new insulating materials for industrial use, for example in capacitors, electrical cables, semiconductors, electronic tubes.
The state of prior art is illustrated by documents [1] to [12] listed at the end of this description.
For the purposes of studying the behavior of insulating materials subjected to strong fields, an understanding of charge injection phenomena within the volume of the material and the associated transport mechanisms is essential. In order to characterize these transient properties, a large number of articles suggest that one face of a sample of insulating material can be charged to a given electrical potential and then the variation of this potential can be monitored with time. The observed decay, called the xe2x80x9cpotential decayxe2x80x9d is a natural phenomenon involving several physical processes such as the injection of charges in volume, polarization or conduction as described in documents [1] and [2]. In this case, it is particularly important to be able to use a process to perfectly control the initial conditions of the decay (quantity and nature of charges, spatial distribution) as to determine the injection and mobility of the charges correctly.
The samples must be previously charged before a potential decay experiment can be carried out. It is usually assumed that this charge is initially close to the surface of the sample. It is very critical to respect this condition in order to study the decay of the potential from its starting point, in other words for a maximum field. Consequently, the charge time must be practically instantaneous compared with the decay time. The potential is usually measured using a slaved potential probe (contact free measurement). Different charging techniques have been used in the past; using the corona effect described for example in document [3], using an electron beam described for example in document [4], or by contact described for example in document [5].
Studies carried out starting from corona discharges have enabled Ieda et al. in document [6] and then other authors later, for example in document [7], to confirm the existence of charges injection into a volume with a high electric field, by indirect effects. However, use of the corona effect is difficult to the extent that it uses a large number of gas ionization and ion deposition processes on the surface of the sample. The nature of the deposited charge and its distribution is then difficult to control. All that can be imposed precisely is the surface potential, without any guarantee about the nature and distribution of the charges. Different combinations of these parameters can give the same surface potential. Furthermore, since the experiment frequently takes place in an ambient atmosphere, a recombination of surface charges with ions in air contributes to the decay, which complicates application of the experiment.
The charge may be directly injected by using a high energy electron beam. With this type of technique, Watson characterized the energy level of traps in which the injected charges are located, in document [4]. More recently, Coelho et al. developed a device in the patent document [8] to measure the mobility of charges injected in an insulating material.
This technique is based on the use of the electron microscope beam to charge the sample. In document [9], Coelho also proposed to use the electrostatic mirror described in patent document [10] for local study of the potential decay on films a few tens of micrometers thick.
The use of an electron beam actually controls the quantity and type of carriers involved. However, the charge is not actually on the surface but is distributed over a thickness that depends very much on electron injection conditions (energy, current, focus, etc.). This thickness is difficult to control.
Furthermore, penetration of electrons imposes the use of samples that are much thicker than the electron stop depth. Consequently, this technique cannot be applied for studying thin layers.
Finally, an excessively high secondary electronic emission can create complex distributions between positive and negative charges. The use of this technique requires thorough knowledge of charge trapping phenomena in insulating materials, which is not always easy to understand.
In order to overcome the problem of electron penetration, charges can be injected by contact with a charged electrode (using an electron beam or a voltage generator). In this case the charge must overcome an energy barrier before penetrating into the material. The result is slower potential decay as described in document [6]. In document [11], Coelho suggested a model to describe this phenomenon. This technique has the advantage that it takes account of the influence of the insulating material/electrode interface in the injection process. This configuration is more representative of electrotechnical applications. It can also be used to study thin layers.
However, when the electron beam is directed directly onto the electrode, the effective energy of the beam reduces as a function of the increase in the potential of the electrode. However, the number of electrons actually remaining on the electrode depends directly on the beam energy. Consequently, the electron beam current can no longer be considered as being constant and may vary considerably during injection until it is cancelled out. The initial potential decay conditions (quantity and distribution of charges) are then not known precisely.
Therefore, regardless of the method used for charging, the quantity and nature of deposited charges are difficult to control satisfactorily. This distorts interpretation of the potential decay and consequently the validity of the associated transport models.
The purpose of the charging process according to the invention is to overcome the disadvantages mentioned above in order to control the quantity and distribution of charges at the end of the charge and therefore at the beginning of the potential decay.
More precisely, the process according to the invention is a process for charging a structure formed from an insulating body sandwiched between two electrodes. It comprises the following steps:
a Faraday cage is placed in contact with one of the electrodes in the structure, the other electrode being made equal to a reference potential;
electrons originating from a controlled electron emission device are introduced into the Faraday cage, the electrons reaching the electrode with which it is in contact in order to charge the structure.
The structure and the Faraday cage can be placed in a vacuum chamber particularly to prevent recombination of electrons participating in the charge with ions in the atmosphere around the structure.
During the charge, the potential of the electrode in contact with the Faraday cage can be measured.
It is preferable to measure a secondary emission of electrons, if any, close to the Faraday cage to make sure that all electrons emitted by the controlled emission device actually participate in the charge.
At the end of the charge, the potential of the electrode in contact with the Faraday cage can be measured at different times, this potential variation representing a potential decay.
This invention also relates to a process for discharging a structure formed of an insulating body sandwiched between two electrodes that were previously charged by the previous charging process, this discharge process comprising a step to short circuit the structure.
The discharge can be obtained by bringing the Faraday cage to the potential of the controlled electron emission device, the reference potential being approximately equal to the potential of the emission device.
A current caused by the discharge when the structure is short circuited can be measured.
The potential of the electrode in contact with the Faraday cage can be measured at different times after the structure is completely discharged.
The present invention also relates to a device for charging a structure formed of an insulating body sandwiched between two electrodes, characterized in that it comprises a controlled electron emission device to inject electrons in a Faraday cage in contact with one of the electrodes in the structure, the other electrode being raised to a reference potential.
It is preferable to put the structure and the Faraday cage inside a vacuum chamber.
The controlled electron emission device may be placed outside the chamber.
The device may comprise a potential probe to make a contact free measurement of the potential of the electrode in contact with the Faraday cage.
The Faraday cage may comprise a solid sidewall, a solid bottom in contact with the electrode of the structure, and at the end opposite to the bottom, a cover in which there is an opening through which electrons from the said controlled electron emission device can arrive.
It is preferable to provide a secondary electron detection device to detect any secondary electrons leaving the Faraday cage through the opening.
The height of the cage from the bottom to the cover is advantageously more than each of its other dimensions to prevent electrons from rising to the diaphragm. This thus improves the trapping efficiency of the Faraday cage.
The area occupied by the Faraday cage on the electrode is advantageously less than the area of the electrode.
The charge device may charge a structure in which the electrode in contact with the Faraday cage is coupled with a arcing horn or electrode field???, and in this configuration it preferably comprises means of bringing the guard electrode up to the same potential as the electrode in contact with the Faraday cage.
A heating and/or cooling device may be provided to adjust the temperature in the vicinity of the structure.
The charging device may be adapted to discharge the structure, and in this configuration it comprises means of short circuiting the structure.
The short circuiting means may make an electrical connection between the Faraday cage and the ground of the controlled emission device corresponding to the reference potential.
The device may then comprise a device for measuring the current caused by discharging the structure.